Inductive position measuring transducers generally comprise transmitter coils, receiver coils, and scale elements. The transmitter and receiver coils may be planar, and arranged on fixed parallel planes, as in a printed circuit board. In some transducers, the transmitter and receiver coils together are called the “readhead.” The scale elements may also be planar and can be coils, bars, or some other shape. Generally the scale elements may alter the magnetic coupling between the transmitter and receiver coils. Generally, the readhead is positioned so that the transmitter and receiver coil planes are parallel to the scale plane. The readhead is movable relative to the scale along the measuring direction. A more complete description of various exemplary prior art inductive transducers can be found in U.S. Pat. Nos. 5,886,519, 6,011,389, RE37,490, U.S. Pat. No. 6,005,387, and U.S. Pat. No. 6,329,813, each of which is hereby incorporated by reference in its entirety.
In such inductive transducer systems, the measurement error of the device generally increases if the alignment between the readhead and scale is not ideal. In particular, a non-zero pitch angle generally leads to imbalances in the signal contributions of various loops of the receiver coils, which is undesirable. An exemplary pitch-compensated transducer that exhibits reduced errors due to pitch misalignment is described in U.S. Pat. No. 5,998,990, (the '990 patent), which is hereby incorporated by reference in its entirety.
One error contribution that can occur due to pitch misalignment is a net imbalance in the signal contributions from the positive and negative loops within a single receiver winding. In order to address related error contributions, the '990 patent teaches a pitch-compensating technique involving flux coupling areas. The '990 patent illustrates planar, spatially modulated, receiver windings that are “twisted” to define effective flux coupling loop areas that alternate their polarity along the measuring axis direction, such that alternating loops provide signal contributions having opposite signs. The '990 patent teaches that the resulting flux coupling areas may be described as being distributed relative to a plurality of hypothetical half-wavelength long polarity zones that alternate in polarity along the measuring axis, corresponding to the scale structure, and corresponding to the alternating polarity of adjacent loops in a receiver coil. The '990 patent teaches that in some pitch-compensated readheads, a receiver winding may have at least one pitch-balancing section designed such that the centroid location, or centroid “axis,” of the effective flux coupling areas making positive polarity contributions to the receiver coil output signal is aligned with the centroid axis of the effective flux coupling areas making negative polarity contributions to that receiver coil output signal. Each spatially modulated section of a receiver coil of the pitch-compensated readhead thus has a defined centroid axis lying in a plane perpendicular to the measuring axis with its location along the measuring axis defined as follows: The location of the centroid axis is defined such that when each incremental portion of all flux coupling areas is multiplied by the signed distance from that incremental portion to the centroid axis, the sum of all such products equals zero. The total effective flux coupling area of the two polarities may also be equal. As a result, a DC signal offset error component may be approximately zero.
However, the foregoing technique, in itself, does not necessarily align the centroid axes of all the various receiver coils, as is most desirable for additional pitch compensation. The '990 patent discusses in detail various types of “pitch error” contributions that can occur in devices which have multiple receiver windings that are generally offset from each other by a spatial phase shift along the measuring axis direction. The '990 patent teaches that if two receiver windings are identical but offset from each other in the measurement direction, then a pitch misalignment brings one receiver winding, on average, closer to the scale than the other receiver winding. Various error contributions may result. For example, the scale is intended to modulate an output signal from each receiver winding, and generally a modulation amplitude mismatch (signal amplitude mismatch) will be created when one receiver winding is closer to the scale (producing a stronger modulation) and another receiver winding is farther from the scale (producing a weaker modulation). This pitch-sensitive signal mismatch may contribute to measurement errors.
In one method for addressing such errors, the '990 patent discloses examples using “multiple loops” (e.g., doubled loops, tripled loops, etc.). In its explanation, in FIG. 20 the '990 patent first shows three windings having a tripled loop configuration wherein the centroids of the various windings are not aligned. The three windings provide three similar signals that have three different spatial phases. That configuration is not pitch compensated for signal amplitude mismatch between windings. Then, in FIG. 21, a configuration is shown that provides similar signals, but that is pitch compensated for signal amplitude mismatch between windings. This is achieved by “rearranging” various loops in the various windings into combinations of single, double and triple loops in various polarity zones, such that the centroids of all the windings are aligned while their desired spatial phases are maintained. Ideally, aligning the centroids effectively cancels the pitch-sensitive mismatch of the signal modulation amplitude between the three windings. Although the relative strengths of the two signal polarity contributions is not necessarily pitch-compensated within a winding for the configuration described with reference to FIG. 21, maintaining pitch-compensated winding-to-winding signal amplitude matching may be the dominant design consideration under many circumstances and, therefore, this type of pitch compensation technique is desirable in various embodiments.
In summary, FIG. 21 illustrates one embodiment of a method for aligning centroids in a multiple loop embodiment to form a pitch-compensated readhead. However, importantly, only an idealized implementation is illustrated, in that the techniques required in a practical implementation for connecting the multiple loops of an individual winding together to form a single output signal are not discussed or addressed. As described further below, in practice, connection fabrication constraints are a dominant design consideration in many applications.
In its embodiments of pitch-compensated readheads, the '990 patent also shows one example of how various portions of a winding may be coupled to one another. As described with respect to FIG. 15, the '990 patent states that first and second receiver windings each have parts for connecting the windings together which are shown as exterior portions extending outside of the transmitter windings. Each of the exterior portions are stated to include two wires that overlay each other to avoid forming a loop. Because the exterior portions do not form loops, it is intended that no signal be generated in these portions by the changing magnetic flux. This solution may be undesirable in certain implementations for a number of reasons, including layout and electrical complications, and increased readhead size. It also does not address the issue of how to form practical connections between multiple closely spaced loops such as those shown in FIG. 21 of the '990 patent.
U.S. Pat. No. 6,124,708, which is hereby incorporated herein by reference in its entirety, discloses a pitch compensated quadrature-type transducer using sets of doubled loops, and also shows a practical method for connecting sets of doubled loops near the edges of two quadrature windings. However, connecting sets of loops near the edges of a winding is a design constraint that may be impractical in various applications. In addition, the pitch-compensation teachings of the '708 patent are difficult or impossible to apply to three phase inductive transducers. Winding configurations which solve any or all of the problems outlined above would be desirable.